Light ends absorption system



United States Patent O LIGHT ENDS ABSORPTION SYSTEM Robert P. Calm and Arthur L. Baron, Elizabeth, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Dec. 2, 1958, Ser. No. 777,697

9 Claims. (Cl. 208--345) The present invention is concerned with the recovery of light end fractions of a hydrocarbon conversion system. More particularly, it deals with an improved method of utilizing isobutane as a lean oil stream in the recovery of light products from a steam cracking reaction.

Various hydrocarbon processes yielding a variety 'of C to C hydrocarbons are well known in the art. Separation of products often proves to be a diflicult, as well as an expensive, operation. This is particularly so when treating the eflluent of a steam cracking reaction zone. Steam cracking reactions give a spectrum of products ranging all the way from H methane, ethylene and ethane to much higher molecular weight olefins, diolefins and aromatics.

Heretofore, when it was desired to recover ethylene and/ or propylene product from the light ends of the steam cracker eflluent, it was customary to pass at least a portion of the steam-cracked product into a primary separation zone operating under conditions adjusted so as to essentially remove C and heavier constituents from its overhead, which then consisted primarily of ethane, ethylene, propane and propylene, and hydrogen, methane diluent. However, since light cracked naphthas are conventionally employed as feed to the top portion of the separation zone so as to scrub out butane and'C and higher constituents, the overhead of the separation zone invariably contains some unsaturated C impurities which must be removed prior to the purification and re covery of the ethylene and/or propylene. Conventionally, this was normally accomplished by a complete sponge oil circuit about a C impurities removal zone, ie. sponge oil absorbing C materials, followed by separating the sponge oil from absorbed C and directly re-using the sponge oil to scrub additional separation zone overheads.

It is further known in the prior art to employ isobutane as a lean oil in recovering ethylene and/ or propylene from the light ends stream after removal of C constituents. Light gases are withdrawn overhead while an isobutane-ethylene/ propylene stream is withdrawn as bottoms. This latter step is advantageous in that since isobutane is a feed material for alkylation reactions, a recovered stream of it and ethylene and/or propylene can directly be used as the feed to an alkylation zone.

However, though considerable effort has gone into improving light ends systems, the methods described above suffer from important disadvantages. In order to prevent loss of isobutane, a valuable hydrocarbon, refrigeration was employed in the operations in which isobutane acted as a lean oil. Temperature control had to be accurately maintained. As is readily appreciated, the expenses of refrigeration as well as the difliculty of precise temperature control present distinct liabilities.

In cases where sponge oil was used to recover isobutane gases which may have passed overhead with light gases, considerable operating and investment cost was incurred due to this second sponge oil circuit required in the overhead system. A combination of an absorber and a stripping or other sponge oil recovery zone was employed in eifecting the absorption treatment of each hydrocarbon stream.

In accordance with the present invention, isobutane may readily be used to recover light ends without incurring the above-described difliculties. Application of isobutane as an absorption oil is combined with the removal of C;,*" constituents from the light ends recovery system in a manner yielding a highly eflicient operation, little or no loss of isobutane, 'and reduced operating and investment costs.

More particularly, the light ends treating zone (normally an absorber-demethanizer) to which isobutane is fed is operated with little or no refrigeration so as to allow a minor but appreciable amount of the isobutane to go overhead with the light gases. The isobutane thus vaporized may be 5-10 mol percent of the light gas overhead. The isobutane-containing overhead is then contacted with a sponge oil having an atmospheric boiling point of at least ZOO-400 F., isobutane being absorbed in the sponge oil while light gases (H C and residual C s) are taken overhead. The mixture of sponge oil and isobutane is then employed to treat the overhead of the primary separation zone (for example, an absorber-depropanizer), C constituents being thus removed in the sponge oil while the isobutane is simultaneously stripped out and returned to the subsequent light ends treating steps, e.g. the absorber-demethanizer, which uses isobutane lean oil, as mentioned before.

By operating in this manner, precise temperature control over the step in which the isobutane serves as a lean oil is not required. No separate stripping operation is needed to recover isobutane since isobutane is returned to the light ends system in the same operation in 'which the sponge oil removes C impurities.

It should clearly be noted that the present specific method of operation ofiers considerably more advantage than one in which the sponge oil-absorbed isobutane mix.- ture is sent to the steam cracker quench or a primary fractionation tower or to the primary separation zone. In that event, the isobutane would be lost to the1C .cut produced in the steam cracking operation, therebydilut ing the C cut as well as increasing the isobutane loss. Further, presence of isobutane in-the steam cracked 1C cut is undesirable for subsequent butadiene extraction and butene utilization. v

. By way of clarifying nomenclature, the expression less than 0.; constituents (used in describing the major portion of the primary separation zone overhead to be treated by the present invention) indicates the substantial absence of butane, butenes, butadiene, etc. 'It may denote streams containing C materials, or streams wherein the only light hydrocarbons present in substantial amounts contain two or less carbon atoms.

The term sponge oil denotes an absorber oil of higher boiling range-lower vapor pressure than the original lean oil, the vaporized portion of which it serves to absorb. v

The various aspects of the present invention will be made more clearly apparent by reference to the follow ing description, drawing and accompanying specific embodiment. f

Referring to the drawing, shown therein is a light ends recovery system operating in accordance with the present invention and comprising basically units 10 through 15. Initially it is to be noted that units 10 and 11 and associated facilities, ie. the C and heavier recovery system, operate substantially in a conventional manner but have been included for the benefit of overall clarity of presen? tation.

Steam cracking, per se, is well known in the art and thus need not be described in detail. Suflice to note that typically a naphtha or gas oil hydrocarbon is elapsed 3 at temperature of 1200 to 1500" F. and in the presence of to 40 wt. percent steam (based on hydrocarbon feed) to yield saturated and unsaturated products, a considerable portion of which comprise C to C hydrocarbons.

After being subjected to quenching in a quench or fractionation tower, steam cracker efiluent is compressed and introduced into a primary separation zone, absorber 10, by means of inlet 45.

In the embodiment described, unit operates as an absorber-depropanizer. Light naphtha, e.g. boiling between C to 430 F., and generally derived from steam cracking, is introduced into the upper portion of depropanizer 10 by line 44. The C and (3 constituents of the steam cracker efliuent are absorbed in the downflowing oil, which is withdrawn from the unit by line 15. A portion of the resulting fat oil is heated by reboiler 17 or the like and recycled by line 16 to unit 10 in order to strip out absorbed C and lighter hydrocarbons from the bottoms product. Absorption can be carried out at a temperature of 35 to 120 F., and a pressure in the range of 100 to 300 p.s.i.g. is employed.

Fat oil containing C materials is passed by line 18 to a conventional debutanizer 11 wherein C constituents are removed as bottoms through line 25 and the C s taken overhead through exit 19. Overhead C gases are cooled to a liquid in condenser 20, condensate being passed to refiux drum 22 by line 21. A portion of the condensate is refluxed to unit 11 by line 23 and the product composed substantially only of C compounds withdrawn by line 24. This 0.; stream may then be sent to cuprous ammonium acetate extraction for the recovery of butadiene or otherwise processed.

Debutanizer bottoms are withdrawn by line 25, heated in unit 27 and a portion thereof recycled by conduit 26 in order to strip C and lighter materials out of the bottom product. Typically, the tower operates at a temperature in the range of 135 to 245 F. The fat oil stream recovered through outlet 28 consists essentially of C and higher hydrocarbons, and is generally passed to other units for chemicals recovery or work-up for inclusion in the gasoline pool. A portion of this stream may be used on the lean oil in absorber-depropanizer 10, as shown by line 46 via cooler 47.

As previously noted, the system described up to this point is generally well known in the art.

The overhead of unit 10 passing through line 29 is composed principally of less than C; constituents, e.g. ethane, ethylene, propane, propylene, along with hydrogen, methane and some acetylenes and propadiene. Due to the vaporization of the light lean oil used in absorber 10, this overhead stream will also contain about 1 to 5, e.g. 2.5 mol percent of C materials, which may be termed to be impurities with respect to the light ends overhead, and are undesirable in the subsequent processing steps.

Thus, in addition to separately recovering various light hydrocarbons from the overhead, it is necessary to remove these C materials. The present invention advantageously serves to accomplish both these steps.

Holding in abeyance the means by which these C impurities are removed (which will be later described in detail), after their removal, the light ends stream is sent to various recovery and conversion zones. In typical cases wherein acetylenes are present in the light overhead, the C stream is passed through conduit 31 to an acetylene conversion zone 32. This generally takes the form of a conventional mild catalytic hydrogenation, the purpose of which is to convert acetylene into ethylene, and propadiene and methyl acetylene into propylene without saturating olefins present in the gas stream. For the purposes of illustration, the hydrocarbons are reacted at a temperature of 350 to 600 F. (depending on catalyst age), at a pressure of 100 to 300 p.s.i.g., eLg. 200 p.s.i.g., and in the presence of a cobalt molybdate or 4 Cr-Ni-Co catalyst. The hydrogen steam cracking product present in the gas stream is usually more than sufficient to provide necessary hydrogen for treating the minor quantities of acetylenes and diolefin.

The thus treated stream is withdrawn through exit 33. In most situations, it is then desired to separate out the C and lighter materials from the C /C constituents which are suitable for feed to an alkylation unit. This is done by means of absorber demethanizer 13.

In order to bring the light hydrocarbon efiiuent of unit 32 to proper conditions for demethanization, they are circulated to compressor 34 by line 33. The compressor serves to raise the pressure of the hydrocarbon stream to the level of demethanizer 13. The compression efiluent stream in line 35 is then cooled in cooler 48 to con dense out water and to reduce the temperature of the hydrocarbon to the temperature of the demethanizer.

As is appreciated by the art, isobutane may be advantageously used as a lean oil in the absorber demethanizer. Whereas in the prior art operating conditions were normally adjusted so as to prevent any vaporization of isobutane lean oil, i.e. refrigeration was applied to maintain a demethanization temperature of 0 to 60 F. at 400 to 700 p.s.i.g. pressure, in accordance with the present invention, refrigeration and the necessity of drying the feed to the absorber-demethanizer to prevent ice formation etc. can be substantially eliminated. Absorberdemethanizer 13 is operated at about 75 to 120 F. top temperature and 120 to 170 F. bottoms temperature (400 to 700 p.s.i.g.), thus allowing a minor fraction of the isobutane fed thereto through line 36 to be taken overhead through line 41 while the bulk of the isobutane absorbs C and C constituents, e.g. ethylene and propylene, and is withdrawn from unit 13 by line 37. In the system illustrated, unit 13 is at 600 p.s.i.g. with an F. top and 145 F. bottoms temperature. Generally, 2 to 10%, e.g. 5%, of the isobutane lean oil will be vaporized and taken overhead.

In the embodiment illustrated, isobutane introduced by line 36 is at a temperature of 85 F. and the light ends of line 35 are at 85 F. About 0.5 to 2.0, e.g. 1.3 mols of isobutane are introduced into the demethanizer for each mol of light ends being fed through line 35.

Normally, at least a portion of the isobutane fat oil, after reboiling in auxiliary reboiler 38, is recirculated to demethanizer 13 by line 39 in order to strip out methane and hydrogen from the bottoms product. The stream withdrawn through exit 40 contains isobutane along with ethylene and propylene and smaller amounts of ethane and propane, and is normally employed as a feed to an alkylation process.

The light ends taken overhead through line 41, containing methane and hydrogen along with the vaporized isobutane, are then passed to absorber 14. Introduced into the upper portion of the absorber through inlet 42 is a sponge oil having a minimum initial boiling point of at least about 200 F. (at atmospheric pressure). The sponge oil may conveniently be a heavy naphtha, kerosene, or light gas oil.

Absorber 14 operates at a temperature of 75 to F. and a pressure in the range of 400 to 700 p.s.i.g. In the embodiment described, it is operated at 85 F. and 600 p.s.i.g., about 0.2 to 0.4 mol, e.g. 0.25 mol, of sponge oil (a 280/360 F.V.T., degree Fahrenheit vapor temperature, oil stream) being used per mol of gases in line 41. The quantity of sponge oil will normally be somewhat in excess of that theoretically required to recover the isobutane since the high value of isobutane requires its complete recovery be attempted, and the sponge oil is to additionally serve as an absorption medium in unit 12. Of course, additional quantities of sponge oil may be added to line 30, by means of line 49 or by means not shown, to supplement the quantity of absorption medium available for absorber 12.

I Methane and hydrogen are taken overhead through line 43 while the sponge oil-isobutane mixture is circulated through line 30 to sponge absorber 12 wherein it is contacted with the overhead of absorber depropanizer 10. Absorber 12 normally operates at 100 to 300 p.s.i.g. pressure and 80 to 120 F. temperature. In the present example it operates at 200 p.s.i.g. and 85 F. The sponge oil (having an atmospheric boiling point of at least 200 F.) serves to condense and absorb the constituents in the depropanizer overhead. Since the overhead of unit has a temperature of at least 90 F. and the pressure of absorber 12 is at most 300 p.s.i.g., isobutane present in the sponge oil is simultaneously stripped out of the mixture of line 30, and is recovered in the overhead of the absorber along with C and hydrogen. If desired, additional sponge oil may be added by line 48. Generally, 0.2 to 0.4 mol sponge oil mixture, e.g. 0.25 mol, is employed per mol of unit 10 overhead.

The isobutane taken from the demethanizer overhead is thus recovered in the same step in which undesired C constituents are removed. The isobutane is circulated through line 31 along with the light ends to the processing steps previously described, ultimately finding its way back to treating zone 13 for further use therein.

Sponge oil containing C impurities is withdrawn from absorber 12 through line 46. Sponge oil may be recovered for further absorption of isobutane by passing the C +-sponge oil mixture to the stream cracking quench tower, to a separate stripping zone or by other means readily apparent to those skilled in the art.

Table 1 indicates the principal components found in the various streams described above.

TABLE 1 Stream compositions Reference Numeral in 44 45 29 31 46 rawmg NNNXNNNNNN O and heavier x Tabulated below is a compilation of data applicable to the practice of the present invention.

TABLE 2 Various modifications may be made to the practice of the present invention. For example, the initial separation zone (unit 10) may be an absorber de-ethanizer when ethylene rather than a propylene/ethylene mixture is desired for alkylation feed. Though the overhead in line 29 will not contain substantial portions of propylene, the isobutane-sponge oil-C removal features will be substantially as previously described. In another modification, unit 13 may act as an alkylation reactor, isobutane and ethylene and/or propylene in the light overhead being reacted in the presence of an alkylation catalyst, e.g. aluminum chloride, under standard alkylation conditions. Methane and hydrogen wil pass through unit 6 13 substantially unconverted, while unreacted isobutane is recovered from the reaction unit overhead in the manner heretofore described. In its broadest aspects, the present invention may be employed in gas cracking, catalytic cracking, and other processes in addition to steam cracking, all these processes being related by their common problem of light ends recovery.

Having described the present invention, that which is sought to be protected is succinctly pended claims.

What is claimed is:

1. In a process wherein an initial stream of relatively light hydrocarbons which contains some C and higher constituents is to be separated into component fractions, wherein said C and higher constituents are removed from said light hydrocarbons by absorption in an oil stream, and wherein isobutane is employed to contact said light hydrocarbons in a treating zone after removal of said C and higher constituents, a minor portion of said isobutane passing overhead from said treating zone, the improved method of separating light hydrocarbons which comprises, contacting the overhead of said treating zone with a sponge oil having a boiling point of at least 200 F. so as to absorb isobutane in said sponge oil, thereafter contacting said sponge oil containing isobutane with said initial stream in an absorption zone so as to vaporize isobutane while absorbing C and higher constituents, removing sponge oil containing C and higher constituents from said absorption zone, and recovering isobutane and light hydrocarbons as the overhead of said absorption zone.

2. The process of claim 1 wherein said initial feed stream is derived from a steam cracking reaction zone, and said treating zone is an absorber-dimethanizer zone, a minor portion of said isobutane passing overhead along with methane.

3. The process of claim 1 wherein said sponge oil contacts the overhead of said treating zone at a temperature in the range of to F., and at a pressure of 400 to 700 p.s.i.g.

4. In recovering the light ends products of a steam cracking reaction wherein an initial hydrocarbon stream predominantly composed of less than C, constituents along with some C impurities is to be separated into light ends fractions, wherein said C impurities are removed by their absorption in an oil stream, and wherein said thus treated hydrocarbon stream is thereafter contacted with isobutane in an absorption zone, the improved method of recovering light ends fraction which comprises; operating said absorption zone so as to allow a minor portion of said isobutane to be withdrawn as part of the overhead of said absorption zone; contacting said absorption zone overhead with a sponge oil having an initial boiling point of at least 200 F. so as to absorb said minor portion of isobutane in said sponge oil; and passing said sponge oil and absorbed isobutane mixture to a C removal zone wherein it contacts said initial hydrocarbon stream, said mixture serving to absorb 0 impurities in said sponge oil while giving up isobutane to the light overhead stream of said C removal zone.

5. The improved method of claim 1 wherein said absorption zone is an absorber-demethanizer zone and about 2 to 10% of the isobutane fed to said zone is vaporized as part of the overhead of said zone.

6. The improved method of claim 4 wherein said initial hydrocarbon stream comprises C C and C constituents recovered from an absorber-depropanizer zone which has been used to treat steam cracking effluent.

7. The improved method of claim 4 wherein said initial hydrocarbon stream is composed primarily of Cf and C constituents derived from an absorber deethanizer zone which has been used to treat steam cracking efiluent.

8. In the process for recovery of light ends from a steam cracking zone wherein steam cracking zone efiluent set forth in the apis treated in an absorber-depropanizer zone, wherein the light ends overhead stream of said absorber-depropanizer zone comprises predominantly less than C, constituents along with some impurities, wherein said light ends stream is treated with an isobutane stream in an absorberdemethanizer zone after removal of C impurities, the improvement which comprises; operating said absorberdemethanizer zone so as to permit about 2 to of said isobutane stream to be withdrawn in the overhead stream of said absorber-demethanizer; contacting said overhead stream of said absorber-depropanizer with a sponge oil having an initial boiling point of 200 F. so as to absorb the isobutane of said stream; passing the resulting mixture of sponge oil and isobutane to a C absorption zone, and contacting said mixture with the overhead of said absorber-depropanizer zone at temperatures of to 120 F. and pressures of to 300 p.s.i.g. so as to absorb C impurities in said sponge oil while isobutane' is freed from said mixture and is' taken overhead along with said predominantly less than C constituents for further treatment.

9. The improvement of claim 8 wherein the overhead stream ofsaid absorber-demethanizer is contacted with said sponge oil at a temperature of 75 to F., and a pressure within the range of 400 to 700 p.s.i.g.

References Cited in the file of this patent UNITED STATES PATENTS 2,324,954 Rupp July 20, 1943 2,439,021 Quigg Apr. 6, 1948 2,710,278 Gilmore June 7. 1955 2,745,889 Johnston et al. May 15, 1956 2,848,522 Gilmore Aug. 19, 1958 2,857,018 Partridge et al. Oct. 21, 1958 

1. IN A PROCESS WHEREIN AN INITIAL STREAM OF RELATIVELY LIGHT HYDROCARBONS WHICH CONTAINS SOME C5 AND HIGHER CONSTITUENTS IS TO BE SEPARATED INTO COMPONENT FRACTIONS, WHEREIN SAID C5 AND HIGHER CONSTITUENTS ARE REMOVED FROM SAID LIGHT HYDROCARBONS BY ABSORPTION IN AN OIL STREAM, AND WHEREIN ISOBUTANE IS EMPLOYED TO CONTACT SAID LIGHT HYDROCARBONS IN A TREATING ZONE AFTER REMOVAL OF SAID C5 AND HIGHER CONSTITUENTS, A MINOR PORTION OF SAID ISOBUTANE PASSING OVERHEAD FROM SAID TREATING ZONE, THE IMPROVED METHOD OF SEPARATING LIGHT HYDROCARBONS WHICH COMPRISES, CONTACTING THE OVERHEAD OF SAID TREATING ZONE WITH A SPONGE OIL HAVING A BOILING POINT OF AT LEAST 200*F. SO AS TO ABSORB ISOBUTANE IN SAID SPONGE OIL, THEREAFTER CONTACTING SAID SPONGE OIL CONTAINING ISOBUTANE WITH SAID INITIAL STREAM IN AN ABSORPTION ZONE SO AS TO VAPORIZE ISOBUTANE WHILE ABSORBING C5 AND HIGHER CONSTITUENTS, REMOVING SPONGE OIL CONTAINING C5 AND HIGHER CONSTITUENTS FROM SAID ABSORPTION ZONE, AND RE- 